The present invention relates generally to an exhaust-gas turbocharger. More specifically, the present invention relates to an exhaust-gas turbocharger having a bearing system located between a turbine and a compressor.
In turbochargers provided with internally-fitted bearing systems, the shaft diameter at the bearing points must be kept small, in order to minimize the frictional losses in the bearings. In order to be able to use inexpensive one-piece bushes as bearings, one end of the shaft must be unoccupied during the assembly operation, meaning that one of the two rotors cannot be attached to the shaft until the latter has been pushed through the bearing bushes, which are not of the split type. This rotor is, as a rule, the compressor impeller, since the turbine wheel is customarily welded to the shaft, because the high operating temperatures to which it is subjected, would prohibit a shrink fit. The attachment of the compressor impeller to the shaft is effected by a shrink fit, or by a press fit which, during repair work, can be released without major difficulty. In addition, this fit facilitates the adjustment, during the assembly operation, of the axial play of a plane surface on the shaft with respect to the bearing. This adjustment permits the required accurate positioning of the compressor impeller in the casing.
In a known design of a press-fit attachment of a compressor impeller to a shaft, an intermediate bush is used. The bush seats, by means of a cone, on the unoccupied end of the shaft and simultaneously serves, by use of an end face, which faces towards the bearing, to limit the axial play of the shaft and to accurately position the compressor impeller with respect to its casing. The compressor impeller sits on this intermediate bush, by a cylindrical press fit. In this design, the shaft-stub, on which the intermediate bush sits, is virtually as long as the intermediate bush itself. The conical surfaces between the shaft and the intermediate bush on the one hand, and the cylindrical press-fitting surface between the intermediate bush and the hub of the compressor impeller on the other hand, extend, at the unoccupied end of the shaft-stub, over only a portion of the length of the hub, preferably over approximately a quarter of this length. A screw, with a washer, provided on the end face of the shaft, serves to secure this attachment axially.
An internal thread, which is provided at the end face of the intermediate bush, and into which a forcing-off screw is inserted, is used in order to pull the intermediate bush from the shaft, together with the compressor impeller.
The arrangement of the cylindrical press-fitting surface for securing the compressor impeller to the unoccupied end of the intermediate bush, that is to say, at the intake end of the compressor impeller, results inevitably from the fact that the hub bore at the rear end of the compressor impeller would expand by an excessive amount, due to the centrifugal force and also, in the case of light-alloy impellers, due to the heating caused by the work of compression, especially at comparatively high pressure-ratios.
The use of an intermediate bush entails the advantage that it can be pulled from the shaft, together with the compressor impeller, as often as desired, without incurring any risk of damaging the attachment.
One disadvantage of this form of construction is that the long shaft-end is, on the one hand, comparatively heavy, while on the other hand its comparatively small diameter renders it insufficiently stiff, to prevent large radial deflections from occurring. Such deflections occur in the event of shockloads and at critical rotation speeds, and necessitate a correspondingly large clearance between the compressor impeller and the casing. In this regard, a particularly dangerous effect is due to the third critical rotation speed, which gives rise to large deflections when the mass is large in proportion to the thin shaft-end, and correspondingly gives rise to large curvatures of the shaft, which can hence come dangerously close to the operating speed. Even small alignment errors in the press fit can act in the same sense. These small errors are capable of giving rise to larger eccentricities of the center of gravity of the compressor impeller, this being particularly true in the case of compressor impellers for high pressure ratios, which are long in the axial direction. A further disadvantage of this form of design resides in the fact that tolerance-related difficulties are created by the coaxial double press-fit within the same longitudinal region.
The present invention, arose from the object of designing the attachment of the compressor impeller to the shaft, in an exhaust-gas turbocharger of the type initially described, in such a manner that the abovementioned disadvantages of the known design are thereby avoided.